SQL选择多对多关系

Question

我有以下架构：

Users
-----
id
name

Conversations
-------------
id
other

Partecipants (join table)
------------
id
user_id
conversation_id
other

一个用户可以有很多对话，并且一个对话属于许多用户。

我需要选择一个用户与其他用户的子集的所有对话。

我的尝试是（不起作用）：

SELECT     *
FROM       `conversations` 
INNER JOIN `participants` ON `conversations`.`id` = `participants`.`conversation_id` 
WHERE      `participants`.`user_id` = 1 
AND         (participants.user_id IN (4,6)) 
GROUP BY    participants.conversation_id 

任何想法？

Answer 1

嗯。 这是使用group by并having ：

select p.conversation_id
from participants p
group by p.conversation_id
having sum(p.user_id = 1) > 0 and     -- user 1 in conversation
       sum(p.user_id in (4, 6)) > 0;   -- user 4 and/or 6 in conversation

Answer 2

从您的问题中我了解到的是，您希望看到与user_id = 1对话的用户“ 4,6”。为此，请尝试执行以下查询。

 select * from (SELECT     conversations.id as conversation_id, participants.user_id as selectedUser,   GROUP_CONCAT(participants.user_id) AS participants
 FROM       `conversations` 
 INNER JOIN `participants` ON `conversations`.`id` = `participants`.`conversation_id` 
GROUP BY    participants.conversation_id ) as derivedTable 
where 
selectedUser=1 and 
(participants like '%4%' and participants like '%6%')

以上查询的作用是。 最初，它将从对话和参与者表中获取所有记录，并再次将所有参与者连接到user_id = 1。 然后外部查询检查很清楚地找到了user_id，参与者有4和6。